[Gambas-user] gb.db.form: Program hangs after erasing content in a DataControl and clicking into a DataBrowser

[Christof Thalhofer]

Hi Tobi,

Am 11.09.2014 um 19:57 schrieb Tobias Boege:

> attached is a project which shows odd behaviour. At first glance, the source
> code seems OK to me, so it might be a bug.
> 
> It needs:
>  - MySQL running, a user named "test" without password but with the ability
>    to create a database.table "Kontakte.kontakte".
> 
> To reproduce:
>  - (If you start the application without the above-mentioned table, it will
>     automatically be created and filled with sample data.)
>  - Select a record,
>  - Go to the DataControl for the "Vorname" field and delete its contents,
>  - Click anywhere into the DataBrowser,
>  - Application hangs.

By a look at "create_table.sql" I see you have not so much sense of
databases ;-)

1st: Use backticks in column names only if you want to name with
"reserved words" like "if", "select" or names with commas or blancs. A
name for a column like "nachname" needs no backticks.

2nd: ("ACHTUNG ... ") Primary key is unique per table. To use "Nachname"
as PK prevents you from inserting "Schmidt" more than one time in a
table. Usually one uses a column with long integer for PK as a unique
number per tuple (datensatz). That means a unique number for example per
customer or invoice.

Now to the code:

    sSQL_Anweisung = "INSERT INTO `kontakte` VALUES "
    sSQL_Anweisung &= "('1','Arno','Adler','Arneburg','39606','Am Hafen
3','03937864322'),"
...

could be eventually dangerous. You should define the columns where you
want to insert your data:

"insert into kontakte (id, vorname, ...) values (1, 'Tobias', ...) (2,
'Albert', ...)"

etc.

Ok, I do not use DataBrowser, so I do not know how it works and why
MySQL hangs. Normally a database "hang" is a deadlock. This means two
connections try to write the same tuple at the same time.

You could see this by increasing verbosity of the database logging and
watching the database logfiles with tail -f ...

It seems that you use 2 connections at the same time in your application
that want to write the same tuple (or table, MyISAM engine  does table
level locking IMHO).

BTW: Table locking is bad (only makes sense for single user). Good
database engines do only row or even column locking for write (or delete).

Remove that code that creates a table from the connect function (that is
really bad). Use one connection which you create in a module (not
class), so you get a singleton and do writing over this one connection
one after the other.

Then you get no deadlocks.


Alles Gute

Christof Thalhofer

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